Data compression systems are well known. Essentially, data compression systems operate on an original data stream, or file, and exploit the redundancies in the data and/or remove superfluous data to reduce the size of the data to a compressed format for transmission or storage. When it is desired to use the data, it is decompressed to a form in which it may be used normally. There are essentially two forms of data compression system, namely reversible (lossless) and irreversible (lossy) systems.
Reversible compression systems are used when it is necessary that the original data be recovered exactly, and these systems are generally used for data such as executable program files, database records, etc. Reversible compression systems include Huffman coding, arithmetic coding, delta modulation, and LZW compression. Depending upon the amount of redundancy in the data (the entropy of the data) to be compressed, reversible compression systems can typically provide a compression ratio of about 2 to 1 or 3 to 1 on real-world images, (expressed as 2:1 or 3:1).
Irreversible compression systems are used when it is not required that the original data be recovered exactly and an acceptable approximation of the data can be employed instead. Unlike reversible compression systems, irreversible compression systems can be designed to provide almost any desired compression ratio, depending only upon the standards to which the recovered approximation of the data is subject.
One common use for irreversible compression systems is image compression, as images generally can undergo irreversible compression and decompression with visually acceptable results. For example, digital still images are often processed with the JPEG (Joint Photographic Experts Group) compression system for storage and/or transmission. The JPEG algorithm is described in many references, including, "JPEG Still Image Data Compression Standard", Van Nostrand Reinhold Publishers, William Pennebaker, Joan L. Mitchell, the contents of which are incorporated herein by reference. Depending upon the intended use for the recovered image, JPEG systems can be set to various desired compression ratios, generally between 2:1 and 40:1, although it should be noted that undesired artifacts of JPEG compression (blocking, moire patterning, "denting & bruising", color quantization etc.) tend to dominate smaller images compressed past 12:1 when using a standard JPEG compression system.
Video images can also be compressed with irreversible compression systems, and the MPEG (Moving Pictures Expert Group) and MPEG-II and MPEG-III compression standards have been proposed as reasonable systems for use in such applications. However, typical undesired artifacts of compression for MPEG include all of the JPEG artifacts plus "glittering" at moving edges and color pulsing. The glittering artifacts are due to pixels with values which are far from a block's mean, shifting the average luminance and/or chrominance of a block from frame to frame as these outlying-valued pixels migrate from block to block due to motion of objects in the scene or motion/zooming of the camera. Also, quantization of the lowest frequency terms in the transform quantize the color gamut irreversibly, so that any subsequent insertion or editing will add to future generations of picture distortion.
Irreversible compression systems trade increased compression ratios for decreased quality of the recovered image i.e.--higher compression ratio, poorer approximation of the data. Unfortunately, at the higher compression ratios requested or required by video providers, the Internet, POTS line users (Plain Old Telephone Service lines) and others, all of the prior art irreversible compression systems known to the present inventor result in recovered images of unacceptable visual quality when required to transmit or store larger images in reasonable bandwidth. A necessary byproduct of irreversible compression is distortion in any statistical sense, but careful choice of processes and filtering criteria can delay the onset of visible degradation well into the "low-statistical-quality" range for PSNR. Similarly, careful attention to minimizing file size using PSNR as the sole criterion can lead to a very high PSNR while the image looks washed out in low visual activity areas such as textures or light patterns.
For example, still 256.times.256 pixel monochrome 8 bit images compressed at a ratio of more than 12:1 with JPEG systems generally exhibit an unacceptable blockiness, which is an artifact of the discrete cosine transform (DCT) block processing stage of the JPEG system. Similar images compressed by a JPEG system to similar ratios present unacceptable "banding" effects and Gibbs phenomena near high contrast boundaries.
Generally, the visible degradations of a recovered image are referred to as compression artifacts and attention has been directed to developing irreversible compression systems whose artifacts are unnoticeable or at least less noticeable to the human visual system at more useful compression ratios, and to developing reversible compression systems with lower entropy resulting from better analytic procedures, more attuned to the Human Visual System (HVS).